1. Field of the Invention
The present invention relates to a focus detecting system, more particularly, one in which two signals corresponding to the image forming states at two positions almost at the same distance before and behind a predetermined focal plane of an image forming optical system, on which plane the object image is to be formed, are obtained. Further, the focus adjusting state of the image forming optical system for the object, particularly, not only to discriminate the in-focus state from the out-of-focus state but also the direction of the out-of-focus state, namely, the so-called near-focus state and the far-focus state, is detected by comparing the above two signals.
2. Description of the Prior Art
Until now, many methods as well as devices for carrying out focus detection by estimating the sharpness of the image formed by the image forming optical system have been proposed. Among them, there is a method in accordance with which, at two positions almost at the same distance before and behind the predetermined focal plane of the image forming optical system photo-electric, elements having characteristics for producing different outputs in correspondence with the variation of the image sharpness are respectively arranged. The difference between the outputs of the two photo-electric elements being used so that the in-focus state is considered to have been obtained when the difference is zero at the time of adjusting the optical system. In accordance with this method, not only can the in-focus state and the out-of-focus state be detected but also the direction of the out-of-focus state, namely the near-focus state and the far-focus state. This is very convenient for constituting a servo system for the optical system in order to carry out the automatic focus adjustment. However, in practice, there is no guarantee that in all cases a superior image forming state could always be obtained. To the contrary, there is a possibility that, in all cases, an inferior image forming state might be obtained, whereby it is impossible to detect the in-focus state correctly so that a mistaken result is obtained.
Particularly, in accordance with this method, the focusing state of the optical system is carried out by comparing the outputs of the two photo-electric elements with each other. It goes without saying that the outputs of both elements are equal to each other when the optical system is correctly focused onto the object. When the optical system is in the considerable out-of-focus state, the outputs of both elements are equal or almost equal to each other, so that it is very difficult to discriminate the true in-focus state from the considerable out-of-focus state, which is the principal weak point of the method.